1. Field of the Invention
This invention relates to gas turbine cooling and particularly to a new and improved air modulation apparatus which permits varying of the amount of air employed for engine cooling to thereby increase engine efficiency.
2. Description of the Prior Art
Many components within gas turbine engines must be cooled in order to maintain those components within acceptable temperature limits. The coolant often used is high pressure air extracted from the engine compressor. However, the remainder of the high pressure air from the compressor is needed for combustion within the engine combustor. The more high pressure air used for cooling, the less there is available to enter the combustor and to perform work in the turbine.
The coolant flow rate within most gas turbine engines is defined by the combustor exit gas temperature which will occur at the highest power setting. However, high power settings are used during only a small percentage of the engine operating time. Thus, engine components, such as the turbine, are adequately cooled during a small percentage of the time and are overcooled the rest of the time. As a result, some of the high pressure cooling air is wasted and the engine must therefore work harder to supply an adequate amount of high pressure air to the combustor. An unmodulated cooling arrangement thus reduces engine efficiency.
Previous air modulation arrangements, which vary the amount of cooling air to engine components, have improved engine efficiency. However, some problems remain. For example, some gas turbine engines employ cooling air circuits which utilize as part of the cooling circuit the channel circumferentially surrounding the rotor shaft which connects the turbine to the compressor. The engines also employ as part of the cooling circuit an annular nozzle, sometimes called a tangential flow accelerator, to introduce the cooling air into the circumferential channel surrounding the rotor shaft. The annular nozzle increases the velocity of the cooling air and turns it to flow in a direction of shaft rotation. Some of the energy of the cooling air invested by the compressor in the process of increasing the pressure of the air is extracted and converted into work by helping to rotate the shaft as the cooling air enters the rotor shaft.
In such previous air modulation arrangements, the cooling air is modulated upstream of the annular nozzle. When the amount of cooling air received by the annular nozzle is reduced, there is a corresponding reduction in the velocity of the cooling air and thus a reduction in the amount of work extracted from the air as it exits the annular nozzle. The turbine then must work harder to turn the shaft. This modulation arrangement can thus reduce engine efficiency.
In view of the above-mentioned problems, it is therefore an object of the present invention to improve the efficiency of gas turbine engines by reducing overcooling of engine components through use of an efficient cooling air modulation apparatus and thereby making available more high pressure air for combustion and turbine work.
Another object of the present invention is to increase engine efficiency by decreasing the ratio of coolant flow to combustor exit flow at operating conditions other than at maximum power setting.
Still another object of the present invention is to provide an air modulation apparatus in which, at a location within the cooling air circuit of a gas turbine engine, the velocity of and thus the work extracted from the cooling air increases as the amount of cooling air supplied to that location is decreased.